1. Field of the Invention
This invention relates to an apparatus for the controlled supply of alumina or other solid materials to an electrolytic tank in which the alumina is converted to aluminium.
2. Description of Related Art
In the electrolysis of alumina, solid alumina is dissolved in a tank or pot containing a molten electrolyte such as cryolite and it is desirable to maintain the alumina concentration in the electrolyte within a predetermined range. In current practice for the electrolysis of alumina, the alumina is fed in successive doses of predetermined size into one or more holes which are made in the electrolyte crust so that the alumina can be admitted when required. As the electrolysis of the alumina proceeds continuously, it would be desirable if the alumina consumed in the electrolysis process could be continuously replaced so as to maintain the optimum alumina concentration in the electrolyte. However, the optimum operating conditions are such that the electrolyte crust continuously reforms on the surface of the electrolyte making it difficult to continuously supply alumina to the molten electrolyte beneath the crust. For this reason, known alumina feeding procedures involve the use of a crust breaker which is operated intermittently to break the electrolyte crust and form a hole through which the solid alumina can be fed. However, the action of the crust breaker is necessarily such that the crust breaking mechanism, such as a pneumatically operated shaft with an appropriate chisel means (hereinafter referred to as a plunger) at its free end, will be moved in and out of the hole formed by the plunger.
In one known feeding procedure, two separate pneumatic systems are employed, one operating the crust breaking mechanism and the other operating the alumina feeding system. In this procedure, it is possible for the mechanism operating the crust breaking mechanism to form the necessary hole in the electrolyte crust and retract the crust breaker so that the feeding system can then be operated to place a charge of alumina into the hole formed by the crust breaker.
In another procedure, a single pneumatic system is used to operate the crust breaking mechanism, and the discharge of alumina from a storage device is co-ordinated with the downward movement of the crust breaker. In this procedure, the alumina charge is thus released when the crust breaker is through the crust so that the alumina is not free to enter the hole in the crust until the crust breaker is retracted. While this procedure has the advantage of a single pneumatic system, it is obvious that not all of the alumina will be able to pass through the hole into the electrolyte immediately when the crust breaker is retracted.